Cine balanced steady state free precession (SSFP) is the cardiovascular magnetic resonance (CMR) technique of choice for assessing ventricular size (end-diastolic volume, end-systolic volume), global function (ejection fraction) and regional function (wall motion, wall thickening) due to its a) high intrinsic blood to muscle contrast that is preserved throughout the cardiac cycle, b) high signal-to-noise ratio, and c) balanced gradient structure in all directions, which provides desirable flow properties [
1-
7]. However, cine SSFP sequences are sensitive to perturbations of the steady state due to field inhomogeneity or bulk–motion (blood flow and myocardial motion) that can cause image artifacts due to initial oscillatory transient signal, and blood to myocardial contrast change during the approach to steady state. In addition, it requires a minimum repetition time (TR) between radio frequency (RF) pulses to reduce banding artifacts. Thus, cine SSFP sequence has conflicting constraints of uninterrupted excitations (single shot), specific absorption rate (SAR) limit affecting the minimum TR, and respiratory motion compensation [
4,
8]. In routine clinical practice, cine SSFP acquisitions are therefore performed during suspended respiration (breath-holding) using a single continuous shot of excitations, and the spatial and temporal resolution are adapted to fit within a single breath-hold session. Conventional cine SSFP techniques typically require 10 to 12 breath holds of 8 to 10 seconds each to cover the entire LV with a temporal resolution of 30 to 45 msec. These successive breath holds are difficult to accomplish in pediatric patients. Even older children may have difficulty holding their breath consistently at the same respiratory level. Younger children who cannot cooperate by lying still for the duration of the study typically undergo general endotracheal anesthesia (GETA) to enable breath-held imaging. The challenges in children, even when intubated, are compounded by the need for higher spatial resolution (small structures) and higher temporal resolution (rapid heart rates) [
9]. There is a growing preference in children’s hospitals towards the use of intravenous sedation rather than GETA for CMR since it is considered to be safe, less invasive and more physiologic, but requires adaptation of the CMR sequences for free-breathing acquisition. A commonly used strategy in this setting is to average the signal over multiple bSSFP acquisitions (multiple NSA or MN) during free-breathing to minimize the effect of respiratory motion [
4]. This approach poses special challenges. Firstly, the respiratory bulk motion makes the tissues being imaged in short-axis orientation to fall in and out of steady state, introducing a potential source of artifacts if the diaphragmatic excursion is significant. Secondly, the relatively large flip angles and short TR in SSFP imaging make cumulative RF dose a source of concern in multi-phase, multi-slice, multi-NSA acquisitions. Thirdly, the through plane motion over several cardiac cycles causes blurring of the endocardial boundary, affecting volumetric measurements. The alternative to multi-NSA acquisition is real time dynamic imaging without cardiac or respiratory synchronization. While this is useful, both these imaging strategies often make tradeoffs in temporal resolution and/or spatial resolution to keep the RF dose within prescribed limits. In addition, while real time imaging makes it possible for qualitative assessment of LV function, quantitative assessment is difficult.
We propose a free breathing respiratory triggered multi-shot cardiac cine SSFP technique (RT-SSFP) with a drive to steady state before each expiration, coupled with arrhythmia rejection and retrospective cardiac gating. The purpose of this study is to compare the RT-SSFP sequence to the multiple number of signal averages (MN-SSFP) technique that is currently used in routine clinical practice for assessment of ventricular function in freely breathing sedated pediatric patients.